1. Field of the Invention
This invention is in the field of strong acids useful for catalysis, and their salts.
2. Description of Related Art
Trifluoromethanesulfonic acid (TFMSA) is used for catalysis where a strong acid is needed. It offers a safer, more easily handled alternative to the inorganic acids, hydrogen fluoride and sulfuric acid, which are widely used in industrial processes. In addition, because of its low molecular weight, trifluoromethanesulfonic acid is relatively volatile, a disadvantage in high temperature processes. Known hydrofluoroalkanesulfonic acids, such as 1,1,2,2-tetrafluoroethanesulfonic acid (TFESA) and 1,1,2,3,3,3-hexafluoropropanesulfonic acid (HFPSA), are made from tetrafluoroethylene and hexafluoropropylene, respectively, and are somewhat higher in molecular weight and therefore have lower volatility, but for still less volatile hydrofluoroalkanesulfonic acids higher molecular weight fluoroolefins would be needed. There are few such fluoroolefins of commercial importance and consequently of ready availability.
Furthermore, salts of TFMSA have utility as photoacid generators and ionic liquids. Those having having a photoactive cationic moiety are useful photoacid generators for microlithography. In the process of microlithography, molecules called “photoacid generators” (PAGs) are used to capture photons of light and generate protons. The PAG plays an important role in the imaging process for both positive- and negative-working chemically amplified resists, because the PAG governs light response properties, such as absorption of light or quantum yield of acid formation. In addition, the PAG governs the properties of the produced acid, such as acid strength, mobility, and volatility.
Useful PAGs include organic onium salts, e.g., iodonium salts and sulfonium salts, with non-nucleophilic anions. Organic onium salts producing trifluoromethane sulfonic acids upon exposure have been particularly preferred, because superior sensitivity and good ultimate resolution of the photoresist system can be obtained. In addition, these PAGs are known to reduce the formation of insolubles (“scum”) on the substrate or at the substrate/resist interface. A known drawback of such PAGs is that minor quantities of the rather volatile trifluoromethanesulfonic acid (TFSA) produced during the irradiation process may evaporate (outgas) from the photoresist film and cause corrosion of the exposure and process equipment. In addition, it is known that resist materials containing PAGs that produce TFSA tend to produce the so-called T-shaped pattern profiles, and show linewidth changes upon process delays due to the high volatility and the diffusion properties of this acid.
In summary, the minor quantities of TFMSA produced from the salt can outgas, causing corrosion to process equipment. Volatility is also related to the rate of diffusion of the acid. Diffusion is an undesirable property in microlithography.
U.S. Pat. No. 6,358,665 discloses onium salt precursors, Y+ASO3-, which generate a fluorinated alkanesulfonic acid. Y represents (R1)(R2)(R3)S+ or (R4)(R5)I+ and A represents CF3CHFCF2 or CF3CF2CF2CF2.
WO 02/082185 discloses photoacid compounds with the general structure R—O(CF2)nSO3X, where n is an integer between about 1 to 4; X is selected from the group consisting of organic cations and covalently bonded organic radicals; and suitable R groups include substituted or unsubstituted C1-C12 linear or branched alkyl groups and substituted or unsubstituted perfluoroalkyl groups.
There is a need for hydrofluoroalkanesulfonates and hydrofluoroalkanesulfonic acids of higher molecular weight, for use in catalysis, in ionic liquids and in photoacid generators. It would be particularly advantageous for such hydrofluoroalkanesulfonates to be preparable from readily available starting materials.